In automatic transmissions with an automated wet starting clutch, in order to activate a mechanical emergency transmission operation function, for example in the event that a transmission control unit has failed, it is usual for the force-flow linkage in the transmission to be eliminated by virtue of the fact that when not energized, the starting clutch is shifted to its disengaged condition. Especially in vehicle conditions typical of flowing traffic, this strategy can lead to critical situations, since the vehicle can only coast forward without any positive propulsion drive.
For technical reasons related to safety, engaging the wet starting clutch in such a driving situation is also not possible, since the force-flow linkage can only be obtained by way of a hydraulic clutch actuation pressure. Inasmuch as when the speed of the vehicle is reduced with the clutch engaged, the vehicle's engine will stall, there still remains some residual vehicle speed, but some important auxiliary aggregates, such as braking force enhancers or steering assistance pumps normally driven by the engine, can no longer be sufficiently powered.
In automatic transmissions with a dry starting clutch, when the mechanical emergency operation function is activated, this clutch is engaged so that the vehicle's drive output remains in driving connection with the engine until the vehicle comes to rest. Although no critical driving situations arise because of this, once the vehicle has stopped, it can no longer be moved or pushed out of the way.
Consequently, there is need for a control device for a starting clutch of an automatic transmission of a motor vehicle which, depending on the engine speed and/or the drive output speed of the transmission, interrupts the force-flow linkage in the drivetrain only when the engine speed or transmission output speed falls below a certain value so that the vehicle's engine does not stall, the auxiliary aggregates remain powered and it is still possible for the driver to drive safely away from any danger area that there may be. In addition, such behavior also allows the vehicle to be moved after coming to rest, since the force flow, between the engine and the transmission, is then interrupted.
Against this background, a hydraulic emergency control system for a belt-type transmission is known from DE 199 43 939 A1 in which a clutch, associated with the transmission, can be disengaged or engaged, depending on the speed of a vehicle drive engine. In this way, in the event of a failure repeated stalling of the drive engine, when the engine speed falls below a certain limit value, can be avoided and starting when the speed rises above a certain value is made possible. Depending on the design of this emergency control system, the engine-speed-dependent control signal can be produced and used as a hydraulic pressure, a pneumatic pressure or an electric voltage.
In addition, a method for controlling an emergency shift program for an automatic transmission with a starting clutch is known from DE 102 38 104 A1, which is especially designed to enable emergency running, even when the vehicle is at rest, and to prevent the engine speed from falling below a stalling threshold. In this method, it is provided that the emergency shift program is actuated by a signal that depends on the vehicle's speed and/or its engine speed. The signal is processed by a valve logic system and has the effect that in thrust operation the force linkage to the engine is interrupted in time to prevent stalling of the vehicle's engine.
Furthermore, a dual-clutch transmission is known from DE 103 38 355 A1, which has a first and a second clutch such that for normal operation to engage/disengage the first clutch, a first hydraulic system and to engage/disengage the second clutch, a second hydraulic system controlled by an electronic system are present. In addition, a status-maintaining hydraulic system is provided to which status signals corresponding to the momentary shift condition of the first and second clutches are passed, via a first and a second hydraulic line, and which is connected by hydraulic control lines to the first and second hydraulic systems. If the electronic system should fail, the status-maintaining hydraulic system controls the first and second hydraulic systems in such a manner that at least in many shift conditions of the two clutches, the shift condition of the clutches that existed immediately before the electronic failure is maintained.
Finally, from DE10 2004 020 569,8 which was not published before the filing date of the present patent application, a control valve arrangement for controlling a starting clutch of an automatic transmission is known with which, in a simple, inexpensively produced and reliable manner, in an emergency control situation the starting clutch can be disengaged if the engine speed and/or the drive output speed of the transmission or the driving speed of the vehicle fall below a predetermined value.
This control valve arrangement comprises a clutch control valve for controlling at least one clutch actuation device which, during normal operation of the transmission, converts a supply pressure delivered to it as a function of a pilot pressure or an electric pilot signal, into a clutch actuation pressure to control the clutch actuation device. The control valve arrangement is also characterized in that to realize emergency transmission operation, if the pilot pressure or the electric pilot signal should fail then, as a function of the engine speed and/or the drive output speed, an activation pressure can be delivered to the clutch control valve or directly to the clutch actuation device, where the clutch is kept in the engaged position so long as the speed remains above a predetermined limiting speed value.
This valve arrangement provides a control device for the emergency driving operation of a vehicle with an automatic transmission that can be produced inexpensively and operated reliably and which is activated, for example when an electronic transmission control unit and/or an electrically actuated clutch control valve fails. The engine-speed-dependent and/or transmission-output-speed-dependent control pressure then ensures that a starting clutch of the automatic transmission remains engaged in order to transmit torque through the transmission so long as the driving speed and thus the speed of the drive engine does not fall below a stalling speed at which the engine's function as a combustion engine would cease.
During such emergency driving operation, if the driving speed falls so much that there is a risk of stalling if the starting clutch remains engaged, then by way of the speed-coupled control pressure, the known control valve arrangement disengages the starting clutch which was until then transmitting torque. This advantageously avoids stalling of the engine so that important auxiliary vehicle aggregates, such as a braking force enhancer and a steering assistance pump, can still be powered by the engine without problems.
Although this known control valve arrangement works very well, it has nevertheless been found that in some operating situations its function is disadvantageous and, therefore, requires improvement. For example, an operating situation of a vehicle can arise in which it has first been moved in a forward or reverse gear with an engine speed above the limiting speed value. In this type of operation, a self-holding valve of the control valve arrangement to be improved will have been actuated by the speed-dependent pressure as described.
Now when starting from this driving operation mode, the vehicle is stopped but the engine speed is still kept above the limiting speed value, the self-holding valve remains in a position such that if the pilot pressure fails, emergency operation with a starting clutch then to be engaged is enabled. However, this system behavior persists when the starting clutch is disengaged by a corresponding selector lever actuation from the forward drive selector lever setting D or from the reverse drive selector lever setting R to the transmission selector lever setting neutral or the parking setting P and in the transmission the most recently used gear remains engaged.
Inasmuch as in such a situation the pilot pressure is absent, the known control valve arrangement engages the starting clutch and the vehicle starts off with a jerk, because of the comparatively high engine speed, although since the selector lever position is at neutral, the vehicle's driver is not prepared for this. For safety reasons, such behavior of the known control valve arrangement should be prevented.
Accordingly, the purpose of the invention is to propose a control valve arrangement of the type in question with which unintentional initiation of an emergency operating function can be avoided.